This invention relates to a method for preparing wetproofed catalyst particles and, more particularly, to a method for non-uniformly depositing polytetrafluoroethylene resin on carbon particles.
Recently it has been proposed to use wetproofed catalyst particles to promote a number of different redox reactions between two fluid phases, at least one of which is an aqueous liquid. An advantage to using wetproofed catalyst particles to promote such reactions is that the wetproofed catalyst particles are not "flooded" by the aqueous liquid phase. This is because coating of hydrophobic material on the catalyst surfaces is accomplished in a discontinuous manner.
Take, for example, the oxidation of liquid sodium sulfide to sodium polysulfide. When introduced, the liquid sodium sulfide reactant will completely surround all of the surfaces of the catalyst but it will not wet the treated portions (those coated with the hydrophobic material). That is, the contact angle between the liquid and the hydrophobic material is so high and the surface adhesion so slight that when the oxidant gas is introduced, it will readily displace the liquid in the wetproofed areas of the catalyst. The adjacent uncoated areas of the catalyst surface will, on the other hand, be wet by the liquid sodium sulfide. This, then, forms the requisite locus of interfacial contact between the gaseous oxidant, liquid sodium sulfide reductant, and catalyst surface. It eliminates the need for the gas to diffuse through the liquid which would otherwise "flood" the surface of the catalyst and thus increases the rate of reaction.
This advantage and others are all discussed in detail in the Smith and Sanders patents, assigned to the assignee of the present invention. Illustrative are Canadian Pat. Nos. 944,535, issued Apr. 2, 1974; 959,628, issued Dec. 24, 1974; 959,821, issued Dec. 24, 1974; and 963,633, issued Mar. 4, 1975.
The use of hydrophobic treated catalyst particles in other contexts is also known. See, for example, Stevens Canadian Pat. Nos. 907,292, issued Aug. 15, 1972, and 941,134, issued Feb. 5, 1974. In the former, Stevens discloses using a catalyst having a coating directly thereon to promote a process for enriching a fluid with hydrogen isotopes. In the latter, a porous hydrophobic support material having catalyst particles thereon is used in such a heavy water production process.
See also, Butler Canadian Pat. No. 700,933 and Fleck U.S. Pat. No. 2,722,504. Butler describes a system for the electrolysis of sodium chloride brine in which the cathode is porous and supplied with oxygen gas to prevent formation or evolution of hydrogen. In one form, the cathode compartment contains a slurry of particulate solids which is agitated by the air stream or by mechanical agitation. The particulate material may be graphite and coated with a hydrophobic material such as polytetrafluoroethylene. The Fleck patent relates to conducting petro-chemical reactions with a solid catalyst material that has a minor amount of silicone resin on its surface. The vapor phase reactions disclosed in Fleck are isomerization, desulfurization, hydrogenation, hydroforming, reforming hydrocracking, destructive hydrogenation and the like.
All of these patentees apparently use a uniform mixture of treated catalysts in terms of the amount of hydrophobic material on the catalyst particles. Thus, several suggest applying a certain weight percent within a range, but none disclose any differential application of hydrophobic material nor is one likely to be achieved absent specialized procedures to do so. This is an important consideration.
As disclosed in the Smith and Sanders patents, a preferred manner of using the wetproofed catalyst particles is to pack them in a fixed bed within a tower reactor. The reactants are then flowed either concurrently or countercurrently through the reactor. The flow characteristics of the reactants through the reactor depend somewhat on the amount of hydrophobic material deposited on the catalyst particles. The catalysts treated with higher weight percent amounts of hydrophobic material offer less resistance to fluid flow than those having been treated with lower weight percent amounts. But, generally the higher the amount of hydrophobic material, the more the catalyst surface is occluded and the less reactive the catalyst.
In the production of polysulfide, for instance, both the activity of the catalyst and flow resistance characteristics are important commercial considerations in terms of the efficiency of the reactor and the amount of polysulfide produced. Accordingly, the need exists for a wetproofed catalyst which will decrease the flow resistance characteristics without significantly decreasing the reactivity -- that is, a wetproofed catalyst which is more efficient than the uniformly treated catalysts of the prior art.